1. Field of the Invention
The present invention relates to a ferritic steel having ultra-fine grains and more particularly to a ferritic steel having an essentially hypoeutectoid composition free of a special alloying element, such as Nb, and having ultra-fine grains in a hot-rolled state. The present invention also relates to a method for producing a ferritic steel having ultra-fine grains.
The ferritic steel herein is a steel in which the major portion of the structure thereof, usually from 70% to 80%, consists of ferritic crystal grains. The ferritic steel may comprise, depending upon the required mechanical propeties, one or more phases other than the ferrite phase, e.g., a pearlite phase, a martensite phase, and/or a retained austenite phase. The ferritic steel may further comprise precipitates such as carbides and nitrides.
2. Description of the Prior Art
The refining of crystal grains is one of the known methods for strengthening steels, enhancing the strength as well as the ductility. This technique is especially important for enhancing the quality of steels used in a hot-rolled state.
Previously, various attempts have been made to produce ferritic steel having a fine ferritic structure. This is because refinement of the crystal grains is the only method available to improve both the yield stress and, thus, the tensile strength, and the ductility, i.e., the rupture transition temperature. In one attempt, the ferritic steel is subjected to a special heat treatment. In other attempts, special alloying elements, such as niobium, titanium, or molybdenum are incorporated into the ferritic steel.
One such method for producing a ferritic steel having ultra-fine grains is disclosed in Japanese Unexamined Patent Publication No. 52-39519. This method aims to produce a non-heat-treated ferritic steel having a No. 13 grain size or higher and which exhibits excellent strength, cold formability, and ductility. In this method, titanium is introduced in an amount of from 0.1% to 0.5% of the steel or 0.5 to 3.5 times the carbon content (Ti/C=0.5 to 3.5) so as to have the titanium or titanium carbides contribute to refinement of the crystal grains and, during hot-rolling, to maintain the draft, at a rolling temperature of 850.degree. C. or less, to 55% or more so as to suppress the growth of ferritic grains. The publication, however, does not teach the importance of the rolling time.
Japanese Unexamined Patent Publication No. 53-95121 discloses a method for producing a high tensile strength steel not using a special alloying element. This method, however, does not relate to the production of a ferritic steel having ultra-fine grains. In this method, the finishing hot-rolling temperature is at least the Ar.sub.3 point so as to obtain a dual-phase mixed structure. After the completion of hot-rolling, rapid cooling is carried out at a temperature ranging from an Ar.sub.3 to Ar.sub.1 temperature to a temperature of 400.degree. C. at the highest. Then, the hot-rolled strip is coiled. Here, the Ar.sub.3 point means the temperature at which steels are transformed from austenite to ferrite during slow cooling from an austenite temperature, and the Ar.sub.1 point means the temperature at which steels are transformed from austenite to pearlite during slow cooling from an austenite temperature. The purpose of rapid cooling before coiling is to increase the hardness of the secondary phases as much as possible and to maintain the volume percentage of the secondary phases as low as possible, thereby improving the mechanical properties. However, under the hot-rolling conditions disclosed in Japanese Unexamined Patent Publication No. 53-95121, fine ferrite grains are not formed. The draft and the rolling time, which are important for refining crystal grains, are not mentioned. Only a method for inducing a dual-phase mixed structure during cooling is disclosed.
The grain size of conventional ferrite steels having fine grains is from over 4 to 6 .mu.m (microns). These ferrite steels are usually produced by means of a method usually referred to as controlled rolling. In controlled rolling, a ferritic steel containing a special alloying element, such as Nb, is heated, prior to hot-rolling, to a high temperature, e.g., 1,200.degree. C. or more, so as to bring niobium or the like into the solid solution of the ferrite matrix. The finishing rolling temperature is 800.degree. C. or less and thus is very low, and hot-rolling is carried out at a heavy draft. In this method, since hot-rolling is carried out after the temperature of a steel strip is lowered, the productivity is considerably decreased and the deformation resistance during the hot-rolling is considerably high. Since the deformation resistance is considerably high, the load applied to a rolling mill is very high, which is disadvantageous from an industrial point of view.
Other methods for producing such ferrite steels include rolling at a low slab heating temperature and forced cooling after hot-rolling. All of these, however, result in a grain size as mentioned above. None can produce, on a industrially applicable basis, crystal grains of 3 to 4 .mu.m size.
On another front, various laboratory methods for producing a ferritic steel having ultra-fine grains have been proposed. In one of these, a nickel containing ferritic steel is repeatedly annealed, thereby alternating the temperature to first more than and then less than the transformation point. Such annealing, however, is obviously impractical industrially.
Thus, there are no conventional methods which make it possible to industrially produce a ferritic steel having a practical hypoeutectoid composition and a grain size of 4 .mu.m or less in a hot-rolled state.